Generally, for increasing the amount of filament per unit volume to downsize a light bulb and elevate an illumination in an illuminated field, methods of arranging multiple coiled filaments closely, and of winding coils double, triple or quadruple have been known, for example. These methods have limitations, however, in increasing the amount of filament per unit volume. Recently, plane form of the coiled filament itself is designed in different forms, such as elliptic and polygonal, rather than circular to increase the amount of filament per unit volume. For example, plane form of the coiled filament may be changed from circular into flat. Alternatively, as disclosed by the Inventor(s) in Coiled filament for Light Bulbs (see Japanese Patent Application Laid-Open No. 2000-82444), a cylindrical coil in plane form, or circular seen from a plane, of the coiled filament can be altered. In this case, the coil rim is bent toward the center of the circle to reduce the plane area. These methods are capable of increasing the amount of filament per unit volume, downsizing a light bulb closer to a spotlight compared to the conventional coiled filament, and improving the radiation efficiency.
As descried above, if plane form of the coiled filament may be shaped flat in plane form, or the rim of the circle in plane form of the coiled filament may be bent toward the center of the circle, it is possible to increase the amount of filament per unit volume compared to the conventional cylindrical coiled filament. This is effective to downsize a light bulb and improve the radiation efficiency to some extent. However, it is desired to develop such a coiled filament that serves in further downsizing the light bulb with higher radiation efficiency.
The present invention is intended to solve the above problems in the art and accordingly has an object to provide a coiled filament having a light emitter with a reduced volume as best possible, which can serve in downsizing a light bulb and elevate an illumination with high efficiency in an illuminated field.